1. Field of the Invention
The present invention relates to a method for manufacturing silicon carbide whisker, and more particularly to a method for manufacturing silicon carbide whisker having a diameter of no shorter than 1 .mu.m
2. Description of the Prior Art
Silicon carbide whisker, being superior to other fibers that are used to make composite materials in strength, elasticity, oxidation resistance, heat resistance, and chemical stability, is attracting a great deal of attention as a composite reinforcement material for ceramics, metals, plastics, etc.
The role expected of silicon carbide whisker in a composite material of which the matrix is either a metal or a plastic resin is to improve the strength, elasticity, and abrasion resistance; whereas the primary effect expected from admixing silicon carbide whisker in a ceramic material is to improve the fracture toughness of the ceramic composite material so that it has a high resistance to breakage.
One of the most important features that are required of whiskers for reinforcing ceramic composite materials is the thickness of the whisker, that is, the mean diameter of the cross section of the whisker. The greater the diameter of the whisker, the greater the fracture toughness of the composite material reinforced by the whisker. Hence, a whisker ought to have a sufficiently large cross-sectional diameter so that a crack caused in the resultant composite material fails to break the whiskers and thus does not develop extensively. For this reason, it is desirable that a silicon carbide whisker used in a ceramic composite material has an average cross-sectional diameter not smaller than 1 .mu.m.
Four methods have been known for manufacturing silicon carbide whisker: (A) crystallizing a whisker from a liquefied silicon carbide under conditions of high temperature and high pressure; (B) dissolving carbon in a molten liquid of metallic silicon, and crystallizing a silicon carbide whisker; (C) crystallizing a whisker by sublimating silicon carbide from silicon carbide powder at a high temperature; and (D) crystallizing a silicon carbide whisker from silicon carbide obtained by means of a pyrolysis of a silicon compound.
Since methods (A) and (B) require extremely high temperatures and high pressures and employ molten metallic silicon, these methods must depend on highly sophisticated manufacturing facilities to avoid difficulty in controlling the manufacturing process. Method (C) also requires extremely high operation temperatures and the operation itself is difficult to control; furthermore the manufacturing facility is complicated and it is difficult to perform fractional collection of the manufactured whisker with precision. Therefore, all of the methods (A) to (C) are not practical for use on a commercial basis.
Currently, therefore, method (D) is predominantly adopted for manufacturing silicon carbide whisker. Variations of method (D) have been proposed. Among them are (1) a method whereby silicon dioxide undergoes a solid phase reduction by means of carbon or by means of metallic silicon and carbon; (2) a method whereby a mixture of a carbonaceous compound and an organic silicon compound or a carbonaceous compound and an inorganic silicon compound is gasified and heated to undergo a reaction; and (3) a method whereby vapor phase crystallization is effected as a reaction off a fluorine=containing silicate with carbon proceeds.
Variations (2) and (3), according to which silicon carbide whisker is grown in a vapor phase, are methods whereby it may be possible to obtain a whisker having a cross-sectional diameter of 1 .mu.m or greater, if the reaction conditions are appropriately selected. However, in the variation (2), since the reaction takes place in the gas phase, the amount of silicon carbide whisker obtained per unit volume of the reaction chamber is extremely small; thus it is necessary to use a huge reaction chamber when the method is practiced on a commercial basis. What is more, since the reaction temperature is high, it is necessary to use a considerable amount of energy to keep the interior of the huge reaction chamber sufficiently hot. Another problem is that the reaction by-product is corrosive. Variation (3) consists of melting a fluorine-containing silicate, adding carbon to the melt to thereby cause deoxidation, and cooling the vapor emerging from the melt to thereby grow silicon carbide whisker. Like variation (2) this method also involves a gas phase reaction so that the same problems exist with variation (b 3) as with variation (2). Besides, since it is necessary to handle a large amount of molten salts, the type of materials appropriate for the manufacturing facility is greatly limited. Another problem with variation (3) is that the molten salts get in the silicon carbide whisker as impurities. Therefore, variation (3) is not a preferred method for making the silicon carbide whisker on a commercial basis.
On the other hand, variation (1), which comprises solid phase deoxidation, uses easily available silicon dioxide as the initial material, with this variation, it is possible to scale up the manufacturing facility without technical difficulties so that the manufacture of silicon carbide whisker on a commercial basis is practical. Studies have been conducted in an attempt to find appropriate sources of silicon atoms to be employed in variation (1). As the result, various sources have been proposed, such as, (a) the silicon fraction obtained from hulls of gramineous plants, (b) quartz sand, (c) shirasu and scrap glass, and (d) active silica having a high specific surface area, e.g., silica sol and silica gel.
L. Patric et al reported that chromium, aluminum, iron, cobalt, copper, silicon, and gold are effective catalysts for the reaction in variation (1) for manufacture of silicon carbide whisker. Proposals were made as to the effective catalysts including those mentioned above. For example, Japanese Kokoku 50-18479, Japanese Kokai 61-22000, and Japanese Kokai 63-159300 teach that besides these metals, effective catalysts include compounds of these metals, and transition metals, such as nickel, and compounds thereof.
However, with the variation (1), it is extremely difficult to obtain a silicon carbide whisker having a diameter greater than 1 .mu.m no matter which one of silicon sources (a) through (d) may be used. If thick whisker of a diameter exceeding 1 .mu.m is ever grown, it occurs only to a limited extent and only in such particular positions such as the surface portion of the raw material layer and gaps therein.
It is also important to reduce the amount of powdery silicon carbide which is created as a by-product together with the silicon carbide whisker. When the whisker contains excessive amount of powdery carbide, the resultant ceramic composite material will exhibit poor fracture toughness. Accordingly, silicon carbide whiskers containing an excessive amount of powdery silicon carbide are often not suitable as a composite reinforcing material for ceramics, metals and plastics. Furthermore, there has been a problem that the powdery products by-produced in the silicon carbide whiskers cannot be completely separated even by a separation method utilizing the difference in the oleophilic and hydrophilic properties or by means of centrifugal precipitation.